Network admin or Sub-site Admin can redirect any user to other website’s portal.
To redirect a user to any other website, navigate to Network Admin > Dashboard > Users.
This will take you to the Users list page. Click on ‘Edit’ link under the user you wish to redirect to other websites portal.
This will redirect you to the user edit page. Navigate to ‘Account Management’ section and choose a website from the dropdown.
Upon next login User will be redirected to the selected sites’ portal.
Note: User Redirection is only for Multisite WordPress Account